An increasingly important technology concerned with the generation, processing, and detection of optical signals that represent electric quantities. Major areas for application of this technology include communications, where digital or analogue information can be transmitted over optical fibres (see fibre optics). A major feature of optical links is their very high immunity to EMI. Another important application is where two electric circuits may exchange signals by means of an optical link yet remain electrically isolated; these devices are called optoisolators. This type of device is used extensively in power supplies and similar applications, where they allow closed-loop control while maintaining a safety barrier to protect users. The technology is also being used to implement logical gating functions employing optical switches, the advantages being high speeds of operation (see optical computing).

Optical signals may be generated from electric signals using transducers such as switchable semiconductor lasers. Detection of optical signals is often achieved using phototransistors, i.e. bipolar transistors whose base drive is made dependent on incident light. See also optical storage.